The TLI Test

The only way to confirm EPI is with a TLI (Trypsin-Like Immunoreactivity) blood test. If your dog is newly diagnosed with EPI, once the dog appears to be stabling out (after starting treatment with enzymes and dog is consistently delivering normal poos) Not necessary, but if possible, talk to your vet about re-testing with another TLI test to eliminate any rare false-positive reads and confirm EPI. The only reason why this is mentioned here is because although almost all dogs that test positive for EPI do indeed have EPI- - there have been extremely rare cases of a false read. Out of over 1,000 dogs...I know of only two that this happened with.

In dogs, values below 2.5 µg/L are diagnostic for EPI. Values between 3.5 and 5.7 µg/L are rarely if ever associated with signs of EPI but may reflect subclinical pancreatic disease such as subtotal pancreatic acinar cell destruction secondary to on-going immune-mediated lymphocytic pancreatitis. Progression of the disease in such patients may ultimately lead to EPI. Values between 2.5 and 3.5 µg/L are sometimes (but rarely) associated with clinical signs due to EPI. In these cases the TLI assay should be repeated after one month paying particular attention to ensuring that food is withheld for 12 to 15 hours before the blood sample is collected.

In cats, values equal to or below 8.0 µg/L are diagnostic for EPI, with values between 8.0 and 12.0 being equivocal. As in the dog, repeating the assay one month later should be considered.

Serum TLI values above 50.0 µg/L (dogs) and 100.0 mg/L (cats) are consistent with either acute or chronic pancreatitis or decreased renal excretion due to severe renal insufficiency, although our experience suggests that serum TLI is often minimally increased even in severe renal failure. Elevated serum TLI concentrations are also seen in some malnourished patients (dogs usually) without evidence of pancreatitis, and in some cats with patchy pancreatic hypertrophy/atrophy (generally considered to be a benign age-related change). Serum TLI is increased in approximately 30-40% of cats and dogs with pancreatitis; it is important to recognize that normal test results do not rule out the possibility of pancreatic inflammation. We believe that in acute pancreatitis testing of samples obtained as soon as possible after the onset of clinical signs is most likely to yield an abnormal test result. If pancreatitis is suspected, a PLI test should be performed. In cats increased serum TLI is often also observed with small intestinal disease. In these cases serum concentrations of cobalamin and folate should be determined for evaluation of the small intestine.

Serum TLI is extremely stable and serum can be shipped at ambient temperatures.

BACKGROUND INFORMATION:

Exocrine pancreatic insufficiency (EPI) occurs as a consequence of insufficient synthesis and secretion of digestive enzymes by the pancreatic acinar tissue. The functional reserve of the pancreas is considerable, however, and EPI only develops when the exocrine secretory capacity is reduced to less than 10 - 15% of normal. At this point residual pancreatic function together with extra-pancreatic mechanisms of digestion cannot support adequate nutrient digestion and so weight loss, diarrhea, and other clinical signs ensue.

ASSAY PRINCIPLE:

Small quantities of zymogens (inactive precursor molecules) of pancreatic proteases are present in the blood of normal animals. Trypsinogen is synthesized exclusively by the acinar cells pancreas, and measurement of this zymogen by assay of TLI provides an excellent indirect index of pancreatic function. This assay detects both trypsinogen and trypsin (hence the use of the term TLI to describe the total concentration of these two immunoreactive species), but the active enzyme (trypsin) is only present in the serum when there is pancreatic inflammation.

SPECIAL CONSIDERATIONS:

Administration of oral pancreatic extracts does not affect serum TLI concentrations in either normal dogs or cats with EPI, so withdrawal of enzyme supplementation prior to testing of dogs and cats that are already receiving supplementation is unnecessary.

Additionally, assays of serum cobalamin (vitamin B12) and folate are strongly recommended whenever serum TLI is assayed. Serum vitamin abnormalities are common in dogs and especially cats with EPI. Therapeutic supplementation may be essential before an optimal response to enzyme supplementation is obtained.

EXPLANATION FOR POSSIBLE TLI VARIANCE: Dr. David A. Williams, developer of the TLI test, said was that there are no absolutes. There is inherent variability in the TLI assay (as with any assay) and this variability is not the same across all values – indeed the variation in the numerical value is greatest for low (especially within the “EPI range”) and high values. So do not over-interpret the absolute values in the “EPI range”. Anything that is reported as less than 2.0 should just be regarded as essentially undetectable. A dog with a reported value of 0.6 is no sicker than one with a value of 1.6, and by the same token, there is no reason to believe that it will need more enzymes or do less well.

Regarding the variability in reported values, the way Dr. Williams explained it to me was that if you took one vial of blood and tested it (for example) on Tuesday at 1:00 and got one numeric value of 1.5… you can take more blood from that same vial and test it on the same equipment on the same Tuesday at 2:00 and get a different numeric value of maybe 1.0. Indeed, if you tested the same sample twice at the same time you still might get the same difference of 0.5 between the two sets of results! The more you did this; took an average of all the values you got from that one vial of blood tested on the same day on the same equipment would yield a more accurate value, but it still would not be all absolute. This is why, he explained, there is variability that does not reflect any significant difference, and both values are diagnostically low and indicative of EPI. Differences between test results of approximately 2.0 and approximately 3.0 are generally reproducible however, and of diagnostic and functional significance. While both are abnormally low values, the former is almost always associated with clinical signs while the latter almost never is. Fortunately, the vast majority of test results are usually clearly diagnostically low (less than 2.5) or clearly normal (greater than 5.0) so these niceties regarding the assay are of no practical consequence. I hope this helps clarify why TLI values may somewhat fluctuate within the EPI range.